Currently, the demand worldwide for wireless services is growing at an ever quickening pace. The demand is not only for an increased number of users but also for extended wireless access capabilities. These capabilities include for example, Internal access, video conferencing and multimedia applications.
Code Division Multiple Access (CDMA) and Wideband-CDMA (W-CDMA) arc spread spectrum based broadband communications technologies that are increasingly being used in mobile wireless communication systems and, in particular, for third generation (3G) mobile systems currently under development. In a CDMA system, multiple users simultaneously occupy the same frequency band. In contrast, each user in a Frequency Division Multiple Access (FDMA) system is assigned a separate frequency. Users in Time Division Multiple Access (TDMA) systems are assigned separate predefined time slots. CDMA systems, however, discriminate between users by using different codes for each user. In the downlink, CDMA base stations simultaneously transmit signals to multiple subscriber mobile stations over a single frequency band whereby each signal is generated using a different code associated with each user. Several advantages of CDMA systems over other multiple access systems such as FDMA and TDMA systems include greatly increased spectral efficiently and the ability to reduce the affects of signal fading by making use of known path diversity techniques.
In a W-CDMA base station, each information signal associated with a mobile station is multiplied by a unique spreading code sequence. The spreading code sequence is formed by concatenating (i.e. multiplying) a particular scrambling code together with a channelization code unique to each mobile station. Note that in W-CDMA, relatively long scrambling codes (38,400 chips) are concatenated with short channelization codes resulting in the spreading code used to modulate the information traffic. Examples of long codes include complex or non-complex PN sequences, for example Gold sequences or maximal length sequences. Examples of short codes include Orthogonal Variable Spreading Factor (OVSF), Walsh or Hadamard codes. Multiplication of the information signal by the spreading code sequence creates the spreading of the signal spectrum by the ratio of the chip rate to the symbol rate. The spread signals for all users are transmitted simultaneously by the base station.
A W-CDMA signal is de-spread by correlating the received signal with the known spreading sequence. The results are adequate as long as the influence of other users can be neglected. The influence between users is reduced by choosing the spreading codes so that the cross correlation between different codes and multipath delays is low (e.g., Gold, OVSF codes).
At each mobile station, a receiver de-spreads the received signal by multiplying the received signal by the code sequence assigned to the receiver. The de-spreading is accomplished using a correlator which functions to generate the information signal intended for the particular mobile station. The correlator operation is such that signals encoded with other user's codes intended for other mobile stations appear at the output of the correlator as minimal noise.
The structure and operation of spread spectrum and CDMA systems are well known. See, for example, Robert C. Dixon, “Spread Spectrum Systems,” John Wiley & Sons, 1984, Andrew J. Viterbi, “CDMA: Principles of Spread Spectrum Communication,” Addison-Wesley Publishing, 1995 and Roger L. Peterson, Rodger E. Ziemer, and David E. Borth, “Introduction to Spread Spectrum Communications,” Prentice-Hall, 1995.
The use of W-CDMA has been proposed for applications requiring higher bandwidth such as multimedia applications. W-CDMA achieves higher data rates by using higher chip rates for the spreading waveform resulting in a higher information rate. Considering that the main interference in a CDMA system is from other users, the increased spreading factor or processing gain allows for improved handling of higher interference levels, which translates to a higher number of users that can be supported in the same cell. In addition, the reduced chip period results in more multipath components being separated by at least one chip period, making more paths available to be resolved by the receiver.
In CDMA systems, rake receivers are typically employed to combat multipath interference. A rake receiver exploits the path diversity present in the input RF signal. The transmitted signal typically travels to the receiver over a channel that includes many independent paths or multipath components. Each multipath component represents a separate route the signal took in traveling from the transmitter to the receiver. A plurality of multipath signals arrive at the receiver with each multipath having a different delay, phase and signal strength due to the fading present in the channel.
Narrowband multiple access techniques such as FDMA and TDMA cannot discriminate between the individual multipath signals that arrive at the receiver because their symbol transmission times are too long, i.e. the symbol duration is relatively long making it impossible to discriminate between individual multipath components. Therefore, these systems must perform equalization in order to combat the negative effects of multipath interference. In CDMA systems, on the other hand, multipath has a minimal negative effect on performance while adding diversity.
The function of the rake receiver in a W-CDMA system is to discriminate between individual multipath signal components, demodulate them and combine them to produce a stronger output signal. Rake receivers typically comprise several ‘fingers’, which are allocated to different multipath components. Each finger is used to receive and demodulate a different multipath component. The energy received from each finger is combined resulting in a stronger received signal. A searcher functions to search for the strongest multipath components and assigns the fingers to those components.
One of the principle tasks of a CDMA receiver and particularly a 3G W-CDMA receiver is the estimation of channel tap delays of the multipath components comprising the input signal. To accomplish this the receiver searches for the channel tap delays associated with different multipath components, i.e. echoes. The search process may be divided into two activities namely (1) detection of new tap delays and (2) tracking of currently known tap delays.
Techniques of channel tap delay estimation are known in the art and typically comprise parallel correlator or serial search based algorithms. In these techniques, a correlator is configured with a set of parameters, including the identity of the code and the relative delay, and adapted to produce a correlation measure between the input signal and a locally reproduced replica thereof. The correlation value generated is used to indicate the existence of a channel tap corresponding to the tested set of parameters.
The channel tap delay estimation is typically performed by the searcher unit in the receiver. The searcher unit is considered the most computationally demanding unit in the receiver and consequently has a major influence on the receiver performance. Thus, an efficient search algorithm has the potential to substantially reduce the computational complexity of the receiver while increasing its performance.
For cellular CDMA systems that operate over a multipath fading channel the problem of estimating tap delays is compounded when considering the requirement of finding and estimating the received delay profile from multiple transmitters. This requirement is related to soft handover and hard handover. In hard handover, the mobile station performs signal reception and demodulation from a single, different, base station than the one(s) before the handover. In soft handover, the mobile station simultaneously performs reception and demodulation of several base stations. For example, both IS-95 and cdma2000 cellular systems utilize a single downlink scrambling code while UMTS W-CDMA utilizes multiple downlink scrambling codes due to an asynchronous network. Thus, there is a need in W-CDMA systems to search over multiple codes and delays that further complicate the search procedure the searcher must perform.
There is thus a need for an efficient search mechanism for performing channel tap delay estimation of the multipath components in an input signal having reduced computational complexity and that permits the receiver to maintain a high level of performance.